#include<iostream>
#include<cmath>
#define S (a+b+c)/2
#define Area(a,b,c) sqrt(S*(S-a)*(S-b)*(S-c))
using namespace std;
int main(){
	double a,b,c;
	scanf("%lf%lf%lf",&a,&b,&c);
	double area=Area(a,b,c);
	printf("%.3lf\n",area);
	return 0;
}
